1. Field of the Invention
The present invention relates, in general, to a ghost cancelling system, and more particularly to a ghost cancelling system for cancelling ghost signals from a video signal using a reference signal of the 8-field sequence type.
2. Description of the Related Art
In recent years, use of a ghost cancel reference (hereafter, GCR) signal for cancelling ghost signals contained in a video signal has become standard in television broadcasting systems. For example, this new GCR method has been adopted as Japan's standard for broadcasting and was introduced in the fall of 1989. The details of GCR signals are given in "Development of a Ghost Cancel Reference Signal for TV Broadcasting", IEEE Trans. on Broadcasting, Vol. 35, No. 4, Dec. 1989, by Miyazawa, Matsuura, Takayama and Usui (hereafter, Ref. 1) which is incorporated herein by reference. The GCR signal described in this Ref. 1 is known as an 8-field sequence type GCR signal. In this system, the ghost signal is cancelled with the aid of a GCR signal inserted with a period of 8 fields in the 18th horizontal scanning line (18H) of the odd numbered fields and the 281st horizontal scanning line (281H) of the even numbered fields. Further, the ghost signal is detected using the finally calculated GCR signal (S.sub.GCR) that is obtained by performing the 8-field sequence calculation indicated by the following equation (1). EQU S.sub.GCR =(S.sub.1 -S.sub.5)+(S.sub.6 -S.sub.2)+(S.sub.3 -S.sub.7)+(S.sub.8 -S.sub.4) (1)
This operation is performed in order to remove the ghost effect of the horizontal scanning line signal previous to the horizontal scanning line in which the GCR signal is inserted, and the horizontal synchronization signal and color burst signal of the horizontal scanning line in which the GCR signal is inserted.
As an example, a conventional ghost cancelling system using a GCR signal of the 8-field sequence type is described in Japanese Patent Disclosure (Kokai) No. 59-211315. In the conventional ghost cancelling system, in order to extract the GCR signals (S.sub.1)-(S.sub.8) corresponding to eight fields, it is necessary to detect the first GCR signal (S.sub.1) itself. However, with such a system, the construction of an S.sub.1 line detector which detects the arrival of the first GCR signal (S.sub.1) is complicated. This is because, in order to detect the first GCR signal (S.sub.1) itself, the following three conditions must all be fulfilled.
______________________________________ condition 1: the horizontal scanning line is the 18H horizontal scanning line; condition 2: the GCR signal is a signal of waveform that rises steeply from black to white, then falls smoothly from white to black (hereafter, WRB (Wide Reverse Bar) waveform signal). condition 3: the phase of the color burst signal is positive polarity (+). ______________________________________
Here, condition 1 can be detected using an ordinary line detection technique. In contrast, detection of conditions 2 and 3 requires complicated processing to ascertain the shapes of the respective waveforms and determine their phase. Therefore, this requirement to detect conditions 2 and 3 made the construction of the conventional S.sub.1 line detection means complicated, making the system costly.